1. Field of the Invention
The present invention relates to a memory decoding system, and particularly to a memory decoding system for use on a portable data terminal.
2. Discussion of the Related Art
A portable data terminal is restricted in its board size, and therefore, all the circuits of the terminal have to be simplified as much as possible. Further, as a dc power source is used, the number of the circuit components (integrated circuit elements) has to be reduced to a minimum in order to prevent the unnecessary consumption of power. The memory decoding system of a portable data terminal includes a separate ROM as the main memory device and a plurality of RAMs together with a microprocessor. These components are provided in such a manner to form the main internal device, internal and external peripheral devices, and a data terminal.
The ROM is primarily used to store the system program, and the plurality of the RAMs are used for carrying out jobs and for operating the programs. Therefore, if the ROM and RAM are assigned with separate addresses, the system program becomes complicated, and can lower the processing performance of the microprocessor owing to the inherent characteristics of the portable data terminal. Therefore, it is required that the ROM and RAM be assigned with addresses in an integrated form.
Generally, the memory decoding system for the portable data terminal uses customized chips as in ASIC (Application Specific Integrated Circuit), and therefore, its construction and operating principles may be difficult to determine. For example, circuit No. 75 which is disclosed in a technical reference manual for Model-20, which is a product of Norland Company of England, includes a memory decoding system for a portable data terminal. The large numbers of gates and logic ICs in the system make it difficult to form compact products.
In that memory decoding system, the memory banks can be selected by a switch capable of three settings for selecting three modes of the memory banks by means of the system program, and the ROM and the RAM are connected to the memory decoding system. However, more detailed exact operating principles of the system cannot be determined.
In forming the memory section as the main memory device for the portable data terminal, if the ROM and the RAM are assigned with independent addresses, as in the conventional cases, the program for operating them becomes complicated, and as a result the processing capability of the microprocessor is reduced. Also, due to the restriction in the board size, all the circuits have to be compactly designed. Further, the number of chips used has to be reduced as much as possible.